The present embodiments relate to an apparatus and also to a method for detecting an antenna coil with a non-active detuning apparatus and also a magnetic resonance tomography system having an apparatus of this kind.
Magnetic resonance tomography systems are imaging apparatuses that, in order to map an examination object, align the nuclear spins of the examination object with a strong external magnetic field and, by a magnetic alternating field, excite the nuclear spins for precession about this alignment. The precession or return of the spins from this excited state into a state with less energy in turn generates, as a response, a magnetic alternating field that is received via antennas.
With the aid of magnetic gradient fields, a spatial encoding is impressed onto the signals, which then permits an assignment of the received signal to a volume element. The received signal is then evaluated, and a three-dimensional imaging representation of the examination object is provided. In order to receive the signal, local antennas (e.g., as local coils) that are arranged directly on the examination object may be used to attain an improved signal-to-noise ratio.
For the receiving, the local coils are set to be resonant at the Larmor frequency (e.g., the Larmor frequency of a hydrogen nucleus in the static magnetic field B0 of the magnetic resonance tomography system). During the excitation pulse for the nuclear spins at the same frequency, however, the local coils are to be detuned in order to avoid a destruction due to high induced voltages or currents. The detuning is effected by active components such as PIN diodes that are made conductive by a control current. If the local coil is not connected, passive protection elements such as crossed diodes or safety fuses are provided. If these protection mechanisms are defective, however, then the resonant conditions of the local coil in the surroundings thereof may lead to excessive field strengths during the excitation pulse, which may possibly endanger a person to be examined despite global limit values generally being observed. This may be the case if there is a particular hazardous situation (e.g., due to implants), and exceeding field strength limit values even for a brief period of time endangers the patient.
A local transmit coil with a monitoring facility is known from the publication DE 10 2015 217 723 A1, for example.